1. Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, Oslo 0316, Norway
2. Lamont-Doherty Earth Observatory, 61 Route 9W, P.O. Box 1000, Palisades, New York 10964-8000, USA
3. Division of Resource Management, Petrified Forest National Park, 1 Park Road, #2217, Petrified Forest, Arizona 86028, USA